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Proper AC wiring not used by FR

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Budward View Drop Down
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Post Options Post Options   Quote Budward Quote  Post ReplyReply Direct Link To This Post Topic: Proper AC wiring not used by FR
    Posted: 21 May 2014 at 5:58am
So-  
Dometic says to use 12 gauge wire for the rooftop AC
NEC says to use 12 gauge wire for a 20 amp breaker.

FR says, screw that we are gonna use 14 gauge and save 50 cents..Angry


I'm digging into the surge currents and voltage drop issues involved with starting a rooftop air conditioner-  it sure doesn't help when FR undersizes the wiring....
For those electrically challenged, undersized wire means more resistance in the wire, that means more power dissipated as heat, which in turns mean less power actually delivered to the rooftop ac especially under high current starting conditions. 
Starting the air is the determining factor in what size generator and its surge capabilities is needed.   A very small generator could run the air, it just can't start it up.

Next step- figure out if FR secures the wiring through the ceiling to the rooftop AC or if I can yank it out and pull proper size wire in....


My Yamaha EF2400isHC is starting my air ok at 300 ft elevation although it struggles a little-  I don't want to be at, say 8000 ft and find it won't, maybe just barely won't because FR undersized the wiring.
I'll be at 5000 ft next week for more testing.
2014 179
Towed by a 2015 Ford Transit Diesel
Supervised by a German/Aussie mix and a Labradoodle!
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techntrek View Drop Down
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Post Options Post Options   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 21 May 2014 at 10:10am
The EF2400is I had last year at altitudes up to 5000 feet started the A/C every time, but there was enough voltage drop during the startup that I felt it was near its limit.  I upgraded to a Honda EU3000is for this year's trip mostly because it has a decently-sized gas tank (the Yamaha only lasted me 8 hours per fillup with the A/C temp set fairly low and the fan set to "on"), but also because this year we'll be at higher altitudes for more of the trip so I didn't want to chance it. 
 
I agree the NEC calls for 12 gauge, that is clearly specified.  However, doing an actual voltage drop calculation shows a difference of 0.7% vs. 1% (10 foot run, 20 amps, 12 vs. 14 gauge).  I wouldn't go to the trouble of pulling new Romex to save 0.3% when both calculated figures are well below the 5% rule.  You could even argue those numbers are lower since a 20 amp circuit is only supposed to run 16 amps continuous.  The startup surge doesn't count since it is allowed to be much higher than the continuous rating.
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Post Options Post Options   Quote Budward Quote  Post ReplyReply Direct Link To This Post Posted: 21 May 2014 at 3:28pm
My figures come out different than yours-  locked rotor amps on the air con is 60+, so lets say it draws 40 amps under startup for a short period, short but very important..
 
I have about 12 ft of wire in the 179 from the breaker panel to the air cond.
 
So 12 ft of 14 has 2.2% voltage drop and 12 ft of 12 has 1.4% at 40 amps.  A difference of .8 %.
Yes nowhere near the 5% target- but then that figure doesn't take into account generators that can barely supply enough to start the air conditioner as is.
 
Am I going to rip the ceiling out to rewire it?  NO!LOL
 
But- if the undersized wire can be fished out while pulling a new wire in without a bunch of trouble then YES I will, heck may even go 10 gauge as long as I'm doing it Smile
 
Point is Dometic specs 12 gauge, I presume they have their reasons, and the NEC specs 12 gauge, but FR didn't do it, proper wiring would be a very minimal expense.
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Post Options Post Options   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 21 May 2014 at 5:02pm
The calculator I use has slightly higher figures @ 40 amps, 2.5% and 1.6%, but again, the surge/LRA isn't part of the usual wire gauge calculations, except in unusual circumstances.  I wouldn't worry about it.  
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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headcold View Drop Down
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Post Options Post Options   Quote headcold Quote  Post ReplyReply Direct Link To This Post Posted: 21 May 2014 at 9:47pm
Techntrek, it doesn't look like my Honda 2000 generator is going to power my AC.  Am I mistaken?  Will it run the convection over?  The microwave?  I think somewhere someone answered these questions for me with a positive ... but given this conversation, I wonder.
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Post Options Post Options   Quote Budward Quote  Post ReplyReply Direct Link To This Post Posted: 22 May 2014 at 3:53am
Originally posted by techntrek

The calculator I use has slightly higher figures @ 40 amps, 2.5% and 1.6%, but again, the surge/LRA isn't part of the usual wire gauge calculations, except in unusual circumstances.  I wouldn't worry about it.  


Correct the surge capacity isn't part of the usual calculations.  In the big picture of electrical standards, this IS an unusual circumstance, considering that probably 99.9% of people in the US will never attempt to run an air conditioner in an RV with a small generator....

Since we don't seem to be on the same page let me reiterate >
 FR did NOT follow the National Electric Code for wiring size required for breaker size.
 FR did NOT follow the air conditioner manufacturers recommendations.
Doing so would have cost almost nothing.


I'm not "worried" about it.  I am disappointed FR did not follow good engineering practice nor did they deserve the touted RVIA "seal" which requires adherence to the NEC.

IF FR had done what they should have done then slightly more starting capability would have been available at the air conditioner.  
Every little bit helps. 
There are even people who have built a timer system that drops the power to the air conditioner fan for several seconds when starting the compressor.
Most of the parts for my fan cutout arrived yesterday and the rest will be in today.
By dropping the fan for a few seconds about 200 to 300 watts used to run the fan is added to the starting capability.
That is a significant amount when using a small generator. 
I need to dig out my Tectronix O'scope and current probe to take some real world measurements...


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Post Options Post Options   Quote furpod Quote  Post ReplyReply Direct Link To This Post Posted: 22 May 2014 at 12:42pm
I have to say, I have camped at 5000 feet and above lots of times. LOTS of times. Never once needed a/c. Needed a snow shovel in August, but not a/c.

and just to keep on topic, we all agree there is 5% of fudge room, and the wiring is under that, correct?

oh, and at the 900 foot or so elevation where I live, a Honda 2000i will start our a/c. We do have a hard start capacitor.
Signature removed, so as not to trigger any non Ford owners, non Irish Setter owners, current R-Pod owners, unhandy or mod negative Pod owners, or single campers of any gender.
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Post Options Post Options   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 22 May 2014 at 12:43pm
I agree 14 gauge wire on that circuit does not follow code, and that they didn't follow the manufacturer's recommendation.
 
My only disagreement is that in this case changing to 12 gauge wire is going to give you any appreciable advantage during startup.  I'm talking physics and not NEC.  The home run is too short.  If this were a 20 amp kitchen countertop circuit and the home run was 75 feet back to the panel, then yes that 12 gauge would be necessary, due to physics and NEC.
 
Headcold - 2000 watts is marginal.  You may find it will start it when it is 75 degrees outside and you are at sea level.  85 at sea level maybe ok, but 95 is too much.  85 at 5000 feet may be too much.  Higher altitude and/or temperature lowers the output rating of a generator (any engine).  Higher temperature increases the internal pressure in the A/C which requires more watts to get it started.  The watts needed goes too high and/or the watts available from the generator is too low and the A/C will stall and the generator will trip its overload protection.
 
furpod - yes, the calculations are within limits.
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Post Options Post Options   Quote headcold Quote  Post ReplyReply Direct Link To This Post Posted: 22 May 2014 at 6:10pm
Good advice.  I don't really need the AC on most of my trips.  I live at 5000 ft and travel to campsites as high as 8000.  But I do need sometimes to run the microwave or the convection oven off the Honda 2000.  I presume from your answers that the AC takes a lot more juice, but that I should be able to run the oven or the microwave on generator power.
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Post Options Post Options   Quote Cap-n-Cray Quote  Post ReplyReply Direct Link To This Post Posted: 22 May 2014 at 8:02pm
I am wiring a surge protector in my 2014 177. I checked the AC and it is on a 20 AMP circuit with 12 gauge wire. All the 15 AMP circuits are 14 gauge. The 30 AMP was 10 gauge.

The only code problem I saw was two ground wires on the same buss bar screw. That is minor. They could strip a little more insulation on the wire for the breaker connections.Disapprove
Cary
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